In the joining of steel with a different metal to produce a clapped steel product it is well known that the effectiveness of the bonding forces between two solid metals is adversely affected by the presence of any contaminating material. P. P. Ewald in a publication titled "Structure and Properties of Solid Surfaces," ed. R. Gomer and C. S. Smith (University of Chicago Press), 1953, disclosed a broad distinction between three kinds of surfaces, namely pure surfaces, clean surfaces and technical surfaces. Ewald described such surfaces in decreasing order of cleanliness as follows:
"(1) Pure Surfaces. The surface of a body created in extreme high vacuum, for example by cleavage, or by evaporator, and maintained in a state where no adsorption occurs. PA1 (2) Clean Surfaces. A surface in the course of whose preparation precautions have been taken to remove grease, moisture, etc., by degreasing and heating in moderate vacuum. PA1 (3) Technical Surfaces. A surface of a body which has been in contact with a reactive environment, such as air, grease or other reagents, whether inadvertently or by design."
In addition to the cleanliness of the bonding surfaces of the metals to be joined it is known that the effectiveness of the bond between two different metals is enhanced by (a) the similarity in their atomic arrangements and (b) the roughness of the bonding surfaces.
Examples of prior art methods of commercial cladding of steel with other metals wherein the cleanliness of bonding surfaces fall into the previously mentioned distinctions of (2) clean surfaces and (3) technical surfaces are as follows:
U.S. Pat. No. 2,691,815 issued to H.W. Boessenkool et al for "Solid Phase Bonding of Metals" is directed to a method comprising the steps of preparing, heating, squeezing and reheating. In this method the initial heating step eliminates "films which would otherwise act as a barrier to bonding" and "the prepared surfaces are protected from reformation of any barrier films" by squeezing.
U.S. Pat. No. 2,753,623 issued to H.W. Boessenkool et al for "Solid Phase Bonding of Metals" is directed to a method comprising the steps of heating and squeezing to form discrete bonds and then sintering. In the method of this invention "barrier films" are removed by heating, wire brushing, pickling and heating in a protective atmosphere prior to squeezing and sintering.
U.S. Pat. No. 3,210,840 issued to J. B. Ulam for "Stainless Steel Clad Aluminum and Method of Making Same" is directed to a method of heating the surface of aluminum and stainless steel and bringing the surfaces into contact with one another under pressure sufficient to effect a reduction in the thickness of the stainless steel and aluminum. This patent discloses mechanically cleaning the surfaces to be joined and heating such surfaces in either a vacuum or an inert atmosphere.
U.S. Pat. No. 3,350,772 issued to J. B. Ulam et al for "Method of Cladding Stainless Steel to Aluminum" is directed to a method of forming a composite stainless steel and aluminum strip comprising heating and exerting pressure to form a bond. This patent discloses an atmosphere selected to protect the clean surfaces of the metal from oxidation by the use of either an inert atmosphere, a reducing atmosphere or a vacuum.
In the prior art cladding of one metal with another metal it is the usual practice to roll bond the metal strips with sufficient pressure to produce extensive elongation, typically more than 30%, for at least one of the metals. Under these prior art conditions it may be possible to obtain partial bonding by providing immediate intimate contact between two surfaces exposed by brittle failure of occluded oxides. However, the limited bonding of the prior art methods of cladding requires additional aids to bonding such as roughening the bonding surfaces to produce high-pressure fracture points and a subsequent heat treatment so as to enlarge isolated bonds by diffusion and/or sintering. Subsequent heat treatments may be harmful to the clad product due to the loss of corrosion resistance caused by carbon migration or the creation of brittle intermetallic compounds such as compounds of iron and aluminum and/or iron and zinc.